The widespread introduction of wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) optical transmission systems relies on the availability of optical transmitters operating at precisely controlled wavelengths. Such transmitters typically use wavelength selected laser diodes as the optical source. Typical DWDM systems operate with many wavelengths, uniformly spaced by frequency, operating in the so-called C-band and/or S-band or L-band, windows of gain provided by the erbium-doped fiber amplifier. For example, in accordance with the optical communications standards set by the International Telecommunications Union (ITU), a DWDM system may operate with 80 channels of different wavelengths uniformly spaced by a channel spacing of 50 GHz. It is anticipated that future systems will operate with greater numbers of channels and with smaller interchannel spacings.
It is also desirable that DWDM systems operate with lasers that are locked to the particular channel frequency, without long-term drift. If the wavelength of the laser drifts, the system may suffer unacceptable crosstalk in adjacent channels. A typical requirement is that the frequency of the laser output does not drift by more than 3 GHz over a span of twenty years. A laser diode will naturally drift by an amount considerably greater than 3 GHz over this time period, the actual amount of the drift being dependent on specific aging characteristics of the laser.
This time-dependent frequency drift can be minimized, if not avoided altogether, by actively controlling the laser wavelength. Active control may include deliberately changing an operating characteristic of the device that affects the output wavelength, such as temperature or current, to compensate for the natural frequency drift. This requires a fixed, known frequency reference for comparison of the emission wavelength from the laser. It is often desirable for network management purposes that each laser be locked locally to its own reference, preferably within the laser diode package. It is also desirable in some circumstances that a single, standard reference assembly can be used with any one of a multitude of fixed frequencies, or with a tunable laser capable of operation at any such wavelength. This enables a widely tunable laser to be used at any of the channel frequencies, and avoids the requirement that the laser be selected to operate within only a small fraction of the channels.
Various wavelength locking solutions have been proposed, including the use of crystal gratings and fiber Bragg gratings, interference filters and etalons. Crystal and fiber Bragg gratings are optimized for operation at one wavelength and do not fit easily into a standard laser diode package. Interference filters can fit inside a laser package, but are typically also optimized for only one wavelength.
Fabry-Perot etalons have been the subject of significant development in wavelength locking schemes. These devices demonstrate a transmission curve that has periodical maxima when plotted against light frequency. This periodical transmission curve needs to be tuned to match the required ITU-grid frequency spacing, which is done either by tilting the etalon or changing its temperature. However, tuning the etalon is a sensitive and complicated process which requires active alignment or precise temperature control. In addition the tuning process becomes more sensitive as the number of ITU channels increases or the interchannel spacing decreases.
Therefore, there is a need for an approach to stabilizing the wavelength of a laser output that is low cost, easily adjustable in production and is sufficiently compact to fit into a standard laser package. Furthermore, since the wavelength locker may be used to stabilize the output from a backup laser diode that substitutes for a laser that has failed, the wavelength locker should be able to operate at any wavelength over the DWDM band.